Patients suffering from unstable, painful knee joints where the instability is caused by the lack or insufficiency of the posterior cruciate ligament may be candidates for a total knee arthroplasty using a posterior-stabilized knee prosthesis. Such an implant or prosthesis includes a tibial component, a femoral component, and typically, a mobile bearing insert (a patella component).
The tibial component is secured to a resected portion of the patient's tibia. This component has a post that receives a bearing or patella component. The femoral component typically has inner intersecting flat surfaces that interface with a surgically prepared distal femoral surface, and an outer surface with a curvature that faces the corresponding tibial component attached to the patient's tibia. The femoral component has two condylar regions with an intercondylar stabilizing housing that accepts the tibial post. These posterior stabilized and constrained femoral implant components have a cam housing in between the condyles of the femur (or the condylar regions).
Bone from the distal femur must be removed from this area to accommodate for this type of cam housing. This may be referred to as cutting the central box geometry. The housing sizes may differ for posterior stabilized and constrained type femoral components. The sizes may also differ from patient to patient and implant to implant. Therefore, a different amount of bone depth may need to be removed, depending upon the patient and the type of femoral implant that will be used.
For optimum results, range of motion, and patient comfort, it is important that the tibial and femoral components are properly aligned relative to the tibia and femur during implantation of the components. There are many accompanying instruments that are used to prepare the femur, e.g., cutting blocks, reamers, saws, chisels, and trial components.
Typically, the posterior bone removed from the distal femur to accommodate the cam housing discussed above is removed using a cutting block and collet configuration. One example is shown and described by U.S. Pat. No. 5,819,829, the entire contents of which are hereby incorporated by reference. The cutting block is placed on the patient's femur with bone spikes and used to make anterior/posterior chamfer cuts. A rotary reamer is then inserted through a collet in the block to prepare the femur for the cam housing (to cut the central box geometry). A box chisel can then be used to complete the preparation. All of this preparation is done using the cutting block as the reference guide.
Some problems encountered with the use of such a system are that once the surgeon has prepared the desired cuts in the femur, s/he will need to remove the cutting block and secure a femoral trial to the prepared distal femur in order to check the fit and sizing. This trialing step is important to ensure that the ultimate component that is implanted fits the area that has been prepared. If additional preparation for the area where the cam housing will fit needs to be done, the trial is removed, the cutting block is re-positioned (which often requires insertion of additional bone spikes) and the cuts are further perfected. It is possible that this procedure may need to be repeated more than once, causing undesired and additional bone removal or degradation during each repositioning step.
Moreover, when the bone is removed to receive the posterior stabilized and constrained femoral implant components through the cutting block, the surgeon does not have a “feel” for how the cuts relate to the actual implant because the cutting block is not shaped like the implant. It is essentially a square block.
Other systems have attempted to cut central box geometry through a femoral trial, but they have not used a constrained box geometry, which can be important to control the reaming instruments and to prevent damage to tissues that are not to be removed. If the trial being used a cutting guide does not have a posterior ledge or stopping portion, it is not always clear where the posterior cutting should stop. Moreover, such systems also fail to provide the desired adjustability that is needed in many cases. If the cutting guide is attached at only a single position with respect to the trial, the surgeon is limited to preparing the box geometry only where the cutting guide sits. See e.g., U.S. Pat. No. 4,721,104, the entire contents of which are hereby incorporated by reference.
Accordingly, there is a need in the art to provide a distal femoral trial that allows a surgeon to cut the central box geometry of a distal femur using an implant-shaped trial, while also providing removability and adjustability of the guide that controls the depth and direction of the cutting instruments. There is also a need for such systems that provide constrained box geometry. There is a further need for surgical methods and kits that use such a trial.